The alarming increase in bacterial resistance to available antibiotics, international travel and newly identified infectious diseases have highlighted the need for new effective vaccines. Inactivated whole-cell vaccines are an important component of the approaches emerging to meet these public health needs. The administration of whole-cell vaccines is one of the most well-studied methods of vaccination against bacterial infection. The particular advantages of whole-cell vaccines include the presentation of many antigens (including protective, but yet undefined antigens), minimal chances of side effects when given non-parenterally, zero virulence potential, and adjuvant-like character. Studies in animal models and humans have shown immunogenicity when whole-cell vaccines were administered orally or parenterally. Effective protection against respiratory, enteric, and systemic bacterial infections has also been shown. Although only inactivated whole-cell pertussis vaccine has been used to immunize the general public, other whole-cell vaccines have the potential for global use.
There are only two basic types of whole-cell vaccines: live attenuated and inactivated. The characteristics of live and inactivated vaccines are different, and these characteristics determine how the vaccine is used.
Live Attenuated Vaccines
Live attenuated vaccines are produced by modifying a disease-producing (“wild”) bacteria in the laboratory. Live attenuated vaccines available in the U.S. include live viruses and live bacteria. These wild viruses or bacteria are attenuated, or weakened, in the laboratory, usually by repeated culturing. In order to produce an immune response, live attenuated vaccines must replicate (grow) in the vaccinated person. A relatively small dose of virus or bacteria is given, which replicates in the body and increases to a volume large enough to stimulate an immune response. Anything that either damages the live organism in the vial (e.g., heat, light), or interferes with replication of the organism in the body (circulating antibody) can cause the vaccine to be ineffective. Although live attenuated vaccines replicate, they usually do not cause disease, such as may occur with the natural (“wild”) organism. When a live attenuated vaccine does cause “disease,” it is usually much milder than the natural disease, and is referred to as an adverse reaction. The immune response to a live attenuated vaccine is virtually identical to that produced by a natural infection. The immune system does not differentiate between an infection with a weakened vaccine bacterium and an infection with a wild-type bacterium. Live attenuated vaccines are generally effective with one dose, except those administered orally.
However, live attenuated vaccines meet with several limitations. First, live attenuated vaccines may cause severe or fatal reactions as a result of uncontrolled replication (growth) of the vaccine virus. This only occurs in persons with immunodeficiency (e.g., from leukemia, treatment with certain drugs, or HIV infection). In addition, depending upon how the vaccine strain was generated, a live attenuated vaccine can sometimes revert back to its original pathogenic (disease-causing) form. To date, this has only been known to occur with live polio vaccine. Active immunity from a live attenuated vaccine may not develop due to interference from circulating antibody to the vaccine virus. Antibody from any source (e.g., transplacental, transfusion) can interfere with growth of the vaccine organism and lead to nonresponse to the vaccine (also known as vaccine failure). Measles vaccine virus seems to be most sensitive to circulating antibody. Polio and rotavirus vaccine viruses are least affected. Live attenuated vaccines are labile, and can be damaged or destroyed by heat and light. They must be handled and stored carefully. Currently available live attenuated vaccines include live viruses (measles, mumps, rubella, polio, yellow fever, vaccinia, and varicella), and two live bacterial vaccines (BCG and oral typhoid).
Inactivated Vaccines
Inactivated vaccines can be composed of either whole viruses or bacteria, or fractions of either. Fractional vaccines are either protein-based or polysaccharide-based. Protein-based vaccines include toxoids (inactivated bacterial toxin), and subunit products. Most polysaccharide-based vaccines are composed of pure cell-wall polysaccharide from bacteria. Conjugate polysaccharide vaccines are those in which the polysaccharide is chemically linked to a protein. This linkage makes the polysaccharide a more potent vaccine. These vaccines are produced by growing the bacteria in culture media, then inactivating it with heat and/or chemicals (usually formalin). In the case of fractional vaccines, the organism is further treated to purify only those components to be included in the vaccine (e.g., the polysaccharide capsule of pneumococcus).
Inactivated vaccines are not alive and cannot replicate. The entire dose of antigen is administered in the injection (as compared to live attenuated vaccines, which provide further “doses” upon replication in the host). Inactivated vaccines cannot cause disease from infection, even in an immunodeficient person. Unlike live antigens, inactivated antigens are usually not affected by circulating antibody. Inactivated vaccines may be given when antibody is present in the blood (e.g., in infancy, or following receipt of antibody-containing blood products). Inactivated vaccines always require multiple doses. In general, the first dose does not produce protective immunity, but only “primes” the immune system. A protective immune response develops after the second or third dose.
In contrast to live vaccines, in which the immune response closely resembles natural infection, the immune response to an inactivated vaccine is mostly humoral. Little or no cellular immunity results. Antibody titers against inactivated antigens fall over time. As a result, some inactivated vaccines may require periodic supplemental doses to increase, or “boost,” antibody titers. In some cases, the antigen critical to protection against the disease may not be defined, thus requiring the use of “whole cell” vaccines. Currently available inactivated vaccines include inactivated whole viruses (influenza, polio, rabies, hepatitis A) and inactivated whole bacteria (pertussis, typhoid, cholera, plague). “Fractional” vaccines include subunits (hepatitis B, influenza, acellular pertussis, typhoid Vi, Lyme disease), toxoids (diphtheria, tetanus, botulinum), pure polysaccharides (pneumococcal, meningococcal, Haemophilus influenzae type b), and polysaccharide conjugates (Haemophilus influenzae type b and pneumococcal).
In summary, it is recognized that the more similar a vaccine is to the natural disease, the better the immune response to the vaccine. While attenuated vaccines are most promising in this regard, they pose risks of disease in immunocompromised hosts and reversion to wild-type, pathogenic organisms. Inactivated vaccines avoid these problems, but yet can be less desirable in that these vaccines do not mimic natural infection and so may not elicit the relevant immune response or elicit as robust, protective an immune response as might be desired.
There is thus a need in the field for safe bacterial vaccines that resemble the infectious organism more closely than the inactivated vaccines, but which have reduced or no significant risk of causing disease in the vaccinated subject. The present invention addresses this need.